It is generally known that certain compounds can upon irradiation with laser light absorb energy from the laser light and are able to transfer this energy to e.g. a matrix polymer the compound is mixed in, thus causing local thermal degradation of the polymer. This degradation may even lead to carbonisation. Carbonisation here is the process that a polymer decomposes due to energy absorption with carbon remaining behind. The quantity of carbon that remains behind depends on the polymer. Many polymers appear not to yield an acceptable contrast upon laser irradiation, be it as such or even when mixed with laser absorbing compounds. From WO 01/0719 it is known to apply antimony trioxide with a particle size of at least 0.5 μm is applied as the absorber. The absorber is applied in polymeric compositions in such content that the composition contains at least 0.1 wt. % of the absorber so as to be able to apply a dark marking against a light background in the composition. Preferably a nacreous pigment is further added to obtain a better contrast.
Also the known composition has the disadvantage that in many cases, in particular in compositions with polymers that in themselves are only weakly carbonising, only a poor contrast can be obtained by laser irradiation. Further antimony trioxide is suspected to be poisonous and there is a need for laser writable compositions not necessarily containing this compound.
The aim of the invention is to provide a composition to which dark markings having good contrast can be written with laser light, even when the matrix polymer is only weakly carbonising or for other reasons is not easily laser writable and can be antimony oxide free.
It has been found that this aim can be achieved in that the composition comprises a polymeric absorber comprising carbonising particles that comprise a core and a shell, the core comprising a carbonising polymer having a first functional group, and the shell, comprising a compatibilising polymer having a second functional group that can react with the first functional group of the carbonising polymer and in that the composition further comprises a reflector.